Chaz is one of your starting characters along with Alys. He was living on the streets until Alys picked him up and taught him the ways of hunting. He's one of the strongest characters in the game, specializing in both swordsmanship and techniques ('magic'). In Final Fantasy terms, he's a Red Mage.
Alys is a famous hunter. She's your typical comic relief/won't do anything for free female lead (like Lina Inverse from Slayers). Although she puts up a tough front, she has a big heart. Her weapon of choice is a battle boomerang. It's a pretty neat attack that hits all on-screen enemies.
Hahn is an assistant at the Piata academy. His idol is Professor Holt, who disappeared when he went to inspect Birth Valley. He joins your battle party before you enter the academy's monster-infested basement. Hahn is not a good character. His attack and defense are low. His arsenal of techniques only consists of basic or low-level skills. When he left my battle party for good, I popped open a bottle of champagne.
Rune is an awesome character! He holds with him a very special past that will eventually be revealed to you towards the end of the game. You're going to love Rune and his powerful techniques & magical spells. However, his defense is low. You might as well equip him with two shields. Rune first joins you early in the game. Then he leaves you! Which is so disappointing. He eventually rejoins you and stays in your party longer.
Gryz lives to avenge his parents, who were killed when the dark wizard Zio destroyed the village of Molcum. The only other surviving member of his family is his little sister, who remains in the village of Tonoe. Gryz relies on brute strength and his skill with the ax. As you may have already guessed, Gryz joins you when you arrive in Tonoe.
Rika is a biological construction created by Seed, one of the last computers on Motavia. She's also a successor to Nei (from Phantasy Star 2)! Rika joins you after your venture through the Bio-Plant. She's a well-balanced character. She's pretty strong with her metal claws and also learns healing techniques.
Demi was created to act as a systems controller for the climate stations built on Motavia. She is the only one who can interface with Nurvus, but has been captured by Zio. She just joins you after you find her in Zio's Castle. Since she is an android, the ways for her to get cured and revived are different than with humans. She has some self-repair skills for herself. If she falls during battle, you can revive her with the Repair-Kit item. If you end a battle with her defeated, she automatically revives and self-restores 1 HP for each step you take when walking.
Wren is an android and the system controller on the Kuran satellite. He feels a sense of responsibility for the problems going on with the system computers, so he decides to join once you meet him. As an android, he has the same perks as I explained above for Demi. Wren is an awesome character! He's strong! You'll love him. He doesn't learn that many techniques and skills, but what he does learn is very useful.
Raja is a sarcastic old priest who protected the Temple near Ryuon until someone (you) had the nerve to land a large spaceship on it. Raja is the best healer in the game. He learns all the best healing techniques the game has to offer.
Kyra is an Esper from the Esper Mansion. You save her after she tries to challenge the Garuberk Tower on her own. I have mixed feelings about Kyra. She's a lot like Hahn where she's a weak 'mage' who only learns mediocre techniques. It is cool, though, that she can equip battle boomerangs to make you feel nostalgic for Alys. And her techniques trigger quite a few combination attacks. Meh, I was happy when she finally leaves your party.